// Copyright (c) 2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file implements BSD-style setproctitle() for Linux.
// It is written such that it can easily be compiled outside Chromium.
//
// The Linux kernel sets up two locations in memory to pass arguments and
// environment variables to processes. First, there are two char* arrays stored
// one after another: argv and environ. A pointer to argv is passed to main(),
// while glibc sets the global variable |environ| to point at the latter. Both
// of these arrays are terminated by a NULL pointer; the environment array is
// also followed by some empty space to allow additional variables to be added.
//
// These arrays contain pointers to a second location in memory, where the
// strings themselves are stored one after another: first all the arguments,
// then the environment variables. The kernel will allocate a single page of
// memory for this purpose, so the end of the page containing argv[0] is the
// end of the storage potentially available to store the process title.
//
// When the kernel reads the command line arguments for a process, it looks at
// the range of memory within this page that it initially used for the argument
// list. If the terminating '\0' character is still where it expects, nothing
// further is done. If it has been overwritten, the kernel will scan up to the
// size of a page looking for another. (Note, however, that in general not that
// much space is actually mapped, since argv[0] is rarely page-aligned and only
// one page is mapped.)
//
// Thus to change the process title, we must move any environment variables out
// of the way to make room for a potentially longer title, and then overwrite
// the memory pointed to by argv[0] with a single replacement string, making
// sure its size does not exceed the available space.
//
// It is perhaps worth noting that patches to add a system call to Linux for
// this, like in BSD, have never made it in: this is the "official" way to do
// this on Linux. Presumably it is not in glibc due to some disagreement over
// this position within the glibc project, leaving applications caught in the
// middle. (Also, only a very few applications need or want this anyway.)

#include "content/common/set_process_title_linux.h"

#include <stdarg.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

extern char** environ;

static char** g_main_argv = NULL;
static char* g_orig_argv0 = NULL;

void setproctitle(const char* fmt, ...)
{
    va_list ap;
    size_t i, avail_size;
    uintptr_t page_size, page, page_end;
    // Sanity check before we try and set the process title.
    // The BSD version allows fmt == NULL to restore the original title.
    if (!g_main_argv || !environ || !fmt)
        return;
    if (!g_orig_argv0) {
        // Save the original argv[0].
        g_orig_argv0 = strdup(g_main_argv[0]);
        if (!g_orig_argv0)
            return;
    }
    page_size = sysconf(_SC_PAGESIZE);
    // Get the page on which the argument list and environment live.
    page = (uintptr_t)g_main_argv[0];
    page -= page % page_size;
    page_end = page + page_size;
    // Move the environment out of the way. Note that we are moving the values,
    // not the environment array itself (which may not be on the page we need
    // to overwrite anyway).
    for (i = 0; environ[i]; ++i) {
        uintptr_t env_i = (uintptr_t)environ[i];
        // Only move the value if it's actually in the way. This avoids
        // leaking copies of the values if this function is called again.
        if (page <= env_i && env_i < page_end) {
            char* copy = strdup(environ[i]);
            // Be paranoid. Check for allocation failure and bail out.
            if (!copy)
                return;
            environ[i] = copy;
        }
    }
    // Put the title in argv[0]. We have to zero out the space first since the
    // kernel doesn't actually look for a null terminator unless we make the
    // argument list longer than it started.
    avail_size = page_end - (uintptr_t)g_main_argv[0];
    memset(g_main_argv[0], 0, avail_size);
    va_start(ap, fmt);
    if (fmt[0] == '-') {
        vsnprintf(g_main_argv[0], avail_size, &fmt[1], ap);
    } else {
        size_t size = snprintf(g_main_argv[0], avail_size, "%s ", g_orig_argv0);
        if (size < avail_size)
            vsnprintf(g_main_argv[0] + size, avail_size - size, fmt, ap);
    }
    va_end(ap);
    g_main_argv[1] = NULL;
}

// A version of this built into glibc would not need this function, since
// it could stash the argv pointer in __libc_start_main(). But we need it.
void setproctitle_init(const char** main_argv)
{
    if (g_main_argv)
        return;

    uintptr_t page_size = sysconf(_SC_PAGESIZE);
    // Check that the argv array is in fact on the same page of memory
    // as the environment array just as an added measure of protection.
    if (((uintptr_t)environ) / page_size == ((uintptr_t)main_argv) / page_size)
        g_main_argv = const_cast<char**>(main_argv);
}
